Post-push fixes: I forgot to include the new files
start_slave.inc and stop_slave.inc in the previuos push.


mysql-test/include/start_slave.inc:
  Added file.
  Purpose: start slave synchronously, waiting for the two
  threads to start.
mysql-test/include/stop_slave.inc:
  Added file.
  Purpose: stop slave synchronously, waiting for the two
  threads to stop.